Blood vessel occlusions are commonly treated by mechanically enhancing blood flow in the affected vessels, such as by employing a stent. Stents act as scaffoldings, functioning to physically hold open and, if desired, to expand the wall of affected vessels. Typically stents are capable of being compressed, so that they can be inserted through small lumens via catheters, and then expanded to a larger diameter once they are at the desired location. Examples in the patent literature disclosing stents include U.S. Pat. No. 4,733,665 issued to Palmaz, U.S. Pat. No. 4,800,882 issued to Gianturco, and U.S. Pat. No. 4,886,062 issued to Wiktor.
FIG. 1 illustrates a conventional stent 10 formed from a plurality of struts 12. The plurality of struts 12 are radially expandable and interconnected by connecting elements 14 that are disposed between the adjacent struts 12, leaving lateral openings or gaps 16 between the adjacent struts 12. The struts 12 and the connecting elements 14 define a tubular stent body having an outer, tissue-contacting (abluminal) surface and an inner (luminal) surface.
Stents are used not only for mechanical intervention but also as vehicles for providing pharmacological therapy. Pharmacological therapy can be achieved by medicating the stents. Medicated stents provide for the local administration of a therapeutic substance at the diseased site. Local delivery of a therapeutic substance is a preferred method of treatment because the substance is concentrated at a specific site and thus smaller total levels of medication can be administered in comparison to systemic dosages that often produce adverse or even toxic side effects for the patient.
One method of medicating a stent 10 involves the use of a polymeric carrier coated onto the surface of the stent 10. A composition including a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend is applied to the stent 10 by immersing the stent 10 in the composition or by spraying the composition onto the stent 10. The solvent is allowed to evaporate, leaving on the surfaces a coating of the polymer and the therapeutic substance impregnated in the polymer.
The dipping or spraying of the composition onto the stent can result in a complete coverage of all stent surfaces, i.e., both luminal and abluminal surfaces, with a coating. However, from a therapeutic standpoint, drugs need only be released from the abluminal stent surface, and possibly the sidewalls. Moreover, having a coating on the luminal surface of the stent can have a detrimental impact on the stent's deliverability as well as the coating's mechanical integrity. A polymeric coating can increase the coefficient of friction between the stent and the delivery balloon. Additionally, some polymers have a “sticky” or “tacky” consistency. If the polymeric material either increases the coefficient of friction or adheres to the catheter balloon, the effective release of the stent from the balloon after deflation can be compromised. Adhesive, polymeric stent coatings can also experience extensive balloon sheer damage post-deployment, which could result in a thrombogenic luminal stent surface. Accordingly, there is a need to eliminate or minimize the amount of coating that is applied to the inner surface of the stent. Reducing or eliminating the polymer from the stent luminal surface also means a reduction in total polymer load, which is a desirable goal for optimizing long-term biocompatibility of the device.
A method for preventing the composition from being applied to the inner surface of the stent is by placing the stent over a mandrel that fittingly mates within the inner diameter of the stent. A tubing can be inserted within the stent such that the outer surface of the tubing is in contact with the inner surface of the stent. A tubular mandrel that makes contact with the inner surface of the stent can cause coating defects. A high degree of surface contact between the stent and the supporting apparatus can provide regions in which the liquid composition can flow, wick, and collect as the composition is applied to the stent. As the solvent evaporates, the excess composition hardens to form excess coating at and around the contact points between the stent and the supporting apparatus. Upon removal of the coated stent from the supporting apparatus, the excess coating may stick to the apparatus, thereby removing some of the coating from the stent and leaving bare areas. Alternatively, the excess coating may stick to the stent, thereby leaving excess coating composition as clumps or pools on the struts or webbing between the struts.
Accordingly, there is a tradeoff when the inner surface of the stent is masked in that coating defects such as pools and clumps can be formed on the stent. There is a need for eliminating or at least minimizing the coating that is formed on the inner surface of the stent as well as coating defects that are formed on the stent struts or between the stent struts caused by the high degree of surface contact between the stent and the mandrel. A mandrel design is needed that addresses these concerns.